This invention relates to a method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive and flowing liquid.
Electromagnetic flowmeters commonly comprise a flow sensor and measuring and control electronics coupled thereto. In the following, only flowmeters or flow sensors will be spoken of if necessary for simplicity.
As is well known, electromagnetic flowmeters measure the volumetric flow rate of an electrically conductive liquid flowing in a pipe; thus, per definitionem, the liquid volume flowing through a pipe cross section per unit time is measured.
The flow sensor has a, usually nonferromagnetic, flow tube which is connected into the pipe in a liquid-tight manner, e.g., by means of flanges or threaded joints. The portion of the flow tube which contacts the liquid is generally electrically nonconductive, so that a voltage induced in the liquid according to Faraday""s law of electromagnetic induction by a magnetic field cutting across the flow tube will not be short-circuited.
Therefore, metal flow tubes are commonly provided with a nonconductive lining, e.g., a lining of hard rubber, polyfluoroethylene, etc., and are generally nonferromagnetic; in the case of flow tubes made completely of plastic or ceramic, particularly of alumina ceramic, the nonconductive lining is not necessary.
The magnetic field is produced by means of two coil assemblies, each of which is, in the most frequent case, positioned on the outside of the flow tube along a diameter of the latter. Each coil assembly commonly consists of an air-core coil or of a coil with a soft magnetic core.
To ensure that the magnetic field produced by the coils is as homogeneous as possible, the coils are, in the most frequent and simplest case, identical and electrically connected in series, so that in operation they can be traversed by the same excitation current. It is also known to cause the same excitation current to flow through the coils alternately in the same and the opposite direction, see U.S. Pat. No. 5,646,353, in order to be able to measure the viscosity of non-Newtonian fluids, i.e., of fluids of high viscosity.
The excitation current just mentioned is produced by control electronics; it is regulated at a constant value of, e.g., 85 mA, and its direction is periodically reversed; this prevents the development of electrochemical interference voltages at the measuring electrodes. The current reversal is achieved by incorporating the coils in a so-called T network or a so-called H network; for the current regulation and current reversal, see U.S. Pat. No. 4,410,926 or U.S. Pat. No. 6,031,740.
The aforementioned induced voltage is developed between at least two galvanic, i.e., liquid-wetted, measuring electrodes, or between at least two capacitive measuring electrodes, i.e., two electrodes disposed in the wall of the flow tube, for example, with each of the electrodes picking off a separate potential.
In the most frequent case, the measuring electrodes are arranged at diametrically opposed positions such that their common diameter is perpendicular to the direction of the magnetic field and, thus, perpendicular to the diameter on which the coil assemblies are located. The induced voltage is amplified, and the amplified voltage is conditioned by means of evaluation electronics to form a measurement signal which is recorded, displayed, or further processed.
The potential at each electrode is not only dependent on the magnetic field according to Faraday""s lawxe2x80x94the geometrical/spatial dimensions of the flow tube and the properties of the liquid enter into this dependencexe2x80x94, but interfering potentials of different geneses are superimposed on this measurement signal, which is based on Faraday""s law and should be as pure as possible.
In principle, the absolute value of the potential at the respective electrode is immaterial for the measurement of volumetric flow rate, but only on condition that, on the one hand, the potentials lie in the dynamic range of a differential amplifier following the measuring electrodes, i.e., that this amplifier must not be overdriven by the potentials, and that, on the other hand, the frequency of potential changes differs appreciably from the frequency of the above-mentioned current reversal.
A first kind of interfering potential results from inductive and/or capacitive interference which arises from coil assemblies and their leads and changes the electric charge on the capacitor that exists at the interface between electrode and liquid. As a result of asymmetries in the concrete structure of the flow sensor, particularly as far as the conductor routing to the coil assemblies and the measuring electrodes is concerned, the interfering potential of one electrode generally differs from the interfering potential of the other electrode.
Thisxe2x80x94firstxe2x80x94effect may, on the one hand, restrict the dynamic range of the differential amplifier. On the other hand, the value of the difference between the interfering potentials of the electrodes is subject to variances in flow-sensor parameters due to manufacturing tolerances. Also, the determinable dependence of the potentials of the measuring electrodes on the velocity of the liquid is partly due to this effect, because at low velocities, the above-mentioned charges at the interface between electrode and liquid are not removed by the latter.
A second kind of interfering potential is caused by particles of foreign matter or by air bubbles which are entrained by the liquid and which, when hitting an electrode, cause sudden changes in the potential of the latter. The decay time of these changes is dependent on the type of liquid and is in any case greater than the transient time of the changes.
Thisxe2x80x94secondxe2x80x94effect, too, results in an erroneous measurement signal. The resulting error is also dependent on the potential of the electrode. Since this potential varies from flow sensor to flow sensor due to manufacturing tolerances as was explained above, the second effect adds to the first effect, so that the individual flow sensor units differ widely in their behaviors, which, of course, is highly undesirable.
A third kind of interfering potential is caused by coatings deposited by the liquid on the measuring electrodes, as is also described in U.S. Pat. No. 5,210,496, for example. The formation of the coatings is very strongly dependent on the velocity of the liquid. The differences in the behavior of the individual flow sensor units are even further increased by the formation of the coatings.
JP-A 10213466 proposes to apply voltages generated by means of the measuring and control circuit to at least one of the two measuring electrodes at least temporarily within a period in which the excitation current does not flow and in which consequently no voltage is induced in the fluid. In the flowmeter disclosed in JP-A 10213466, the temporarily applied voltage serves to measure and evaluate the interface capacitances built up at the measuring electrodes, i.e., in effect to compensate interfering potentials of the first and/or third kinds.
A disadvantage of the prior-art flow sensors is that because of the intermediate turning off of the excitation current, the flow rate can only be sampled very coarsely.
It is an object of the invention to provide a method whereby the aforementioned interfering potentials can be prevented as effectively as possible or at least be largely compensated, and which simultaneously permits high-resolution sampling of the flow rate.
To attain this object, the invention provides a method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of:
causing the fluid to flow through the flow tube;
causing a, particularly bipolar, excitation current generated by means of a measuring and control circuit of the flowmeter to flow through a coil assembly mounted on the flow tube for producing a magnetic field cutting across the fluid;
inducing a voltage in the moving fluid for changing potentials applied at measuring electrodes positioned at the flow tube;
removing potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid; and
applying at least intermittently a discharge voltage generated by a measuring and control circuit to at least one of the measuring electrodes during the flow of the excitation current through the coil assembly.
Furthermore, the invention provides an electromagnetic flowmeter for a fluid flowing in a line, comprising:
a flow tube connectable into the line for conducting the fluid;
a measuring and control circuit;
means fed by the measuring and control circuit for producing a magnetic field cutting across the flow tube with a coil assembly mounted on the flow tube and traversed by an excitation current;
at least two measuring electrodes for picking off potentials which are induced in the fluid flowing through the flow tube, which fluid is penetrated by the magnetic field;
means connected at least intermittently to the measuring electrodes for generating at least one measurement signal derived from the potentials induced in the fluid; and
means controlled by the measuring and control circuit for generating voltage pulses at the measuring electrodes, said means for generating voltage pulses being controlled such that the voltage pulses are applied to the measuring electrodes when the excitation current is different from zero.
In a first preferred embodiment of the method of the invention, the discharge voltage is an at least intermittently periodic sequence of voltage pulses.
In a second preferred embodiment of the method of the invention, the excitation current is at least intermittently clocked periodically with a predeterminable period, with the period of the clocked excitation current being different from a period of the sequence of voltage pulses.
In a third preferred embodiment of the method of the invention, the period of the clocked excitation current is greater than the period of the sequence of voltage pulses.
In a fourth preferred embodiment of the method of the invention, two successive voltage pulses are of like polarity.
In a fifth preferred embodiment of the method of the invention, two successive voltage pulses are of different polarity.
In a sixth preferred embodiment of the method of the invention, the voltage pulses have a width substantially smaller than a pulse width of the excitation current.
In a seventh preferred embodiment of the method of the invention, voltage pulses are applied intermittently to both measuring electrodes.
One fundamental idea of the invention is to largely prevent or compensate interfering potentials of the kinds described, on the one hand, by repeated application of the discharge voltage, particularly at as high a clock rate as possible, and, on the other hand, by constant reversal of the polarity of the discharge voltage, and, despite this discharge process, except for the usual reduction in dynamic range due to the polarity reversal of the magnetic field, to have to trade off no or only a minimum reduction in the sampling frequency of the measurement voltage induced in the fluid. Another fundamental idea of the invention is to apply the discharge voltage to the measuring electrodes at a rate of change, i.e., a repetition rate, higher than the rate of change of the interfering potentials, thus causing charges on the interface and/or stray capacitances which on a time average are substantially uniform.
One advantage of the invention is that the interfering potentials resulting from inductive/capacitive interference, the interfering potentials resulting from particles of foreign matter and/or air bubbles entrained by the liquid, and the interfering potentials resulting from the formation of measuring-electrode coatings can be selectively reduced or even eliminated, so that the wide manufacturing spread can be minimized and flow sensor units of uniform behavior can be manufactured. Another advantage of the invention is that despite the discharge processes initiated at the measuring electrodes, flow rate can be measured with a high resolution in comparison with that of conventional flowmeters.